1. Field of the Invention
The present invention relates to a processor and processing method for an image signal, an image display apparatus, a generation apparatus and generation method for coefficient data used therein, a program for executing each of these methods and a computer-readable medium which records the program.
2. Description of Related Art
As a system for compression-encoding an image signal, there is an encoding system by means of moving picture experts group 2 (MPEG2) using discrete cosine transform (DCT) operation.
The DCT operation executes discrete cosine transform onto pixels within a block, and re-quantizes the coefficient data obtained as a result of the discrete cosine transform, and then further executes variable-length encoding onto the re-quantized coefficient data. In many cases, an entropy encoding such as Huffman encoding is used as the variable-length encoding. The image data is subject to orthogonal transform, thereby dividing it into a multitude of items of frequency data ranging from low frequency to high frequency.
When the divided frequency data is re-quantized, since low-frequency data has high importance taking into consideration of the visual characteristics of human beings, such low-frequency data is quantized in details. Contrarily, since high-frequency data has low importance taking into consideration the visual characteristics of human beings, such high-frequency data is roughly quantized. In this manner, high image quality is maintained and compression is realized with high efficiency.
Decoding by use of a conventional DCT operation converts quantized data for each frequency component into a typical value of its code, and executes inverse DCT (IDCT) operation for these components so as to obtain reproduced data. At the time of transformation into the typical value, a quantized step width at the time of encoding is used.
In addition, the encoding system by means of MPEG2 executes motion-compensated predictive encoding.
As described above, the encoding method by means of MPEG by use of DCT operation executes encoding taking into consideration the visual characteristics of human beings. In this manner, high quality is maintained and compression is realized with high efficiency.
However, since the encoding which executes DCT operation is a processing performed in the unit of blocks, there may occur noises in a form of block, that is, block noises (block distortion) as the compression rate increases. In addition, at a portion having a sudden change in brightness such as an edge, a mosquito noise resulted from roughly quantizing high frequency components occurs.
Such an encoding noise (encoding distortion) may occur not only in the encoding system by means of MPEG2, but also in other encoding systems.
The encoding system by means of MPEG 2 executes motion-compensated predictive encoding. As is already known, the encoded data of MPEG2 is expressed in a hierarchy structure. The hierarchy structure consists of a sequence layer, a group of picture (GOP) layer, a picture layer, a slice layer, a macro-block layer, and a block layer, starting from the high-order layer in this order.
The group of picture (GOP) layer starts from a GOP header, and normally consists of 10 to 15 pictures. The front picture is always an intra-picture (I-picture). The encoding structure of MPEG2 includes, in addition to the I-picture, a predictive-picture (P-picture) and a bidirectionally predictive-picture (B-picture).
The I-picture is an image obtained as a result of in-frame/in-field encoding, and is encoded independently from other frames/fields. The P-picture is an image obtained as a result of intra-frame/intra-field encoding based on a forward prediction from the I-picture and P-picture which are past in terms of time. The B-picture is an image obtained as a result of intra-frame/intra-field encoding based on a bidirectional prediction.
The units of the predictive encoding of MPEG2 are classified into two kinds: a frame picture unit and a field picture unit. When the frame picture unit is selected, the frame produced from an interlaced image is used as a picture unit for motion-compensated predictive encoding. When the field picture unit is selected, two most-recently encoded fields are used as a picture unit for motion-compensated predictive encoding.
The motion-compensated prediction of frame picture can be executed by use of any one of three prediction modes, that is, 1) a frame motion-compensated prediction, 2) a field motion-compensated prediction, and 3) dual-prime prediction. The motion-compensated prediction of field picture can be executed by use of any one of three prediction modes, that is, 1) field motion-compensated prediction, 2) dual-prime prediction, and 3) 16×8 motion-compensated prediction.
The motion-compensated predictive encoding subtracts each pixel data of a reference block which has been motion-compensated based on the motion vector from each pixel data constituting the input image block, and executes DCT operation onto the residual data remaining after the subtraction. In this case, the motion vector has an accuracy of ½ pixel.
For this reason, when the motion vector has a component of ½ pixel, pixels with integer accuracy are averaged to obtain pixel with an accuracy of ½ integer, and in turn to obtain a reference block. Therefore, when the motion vector has a component of ½ pixel, each pixel data in the reference block has a decreased number of high frequency components. The residual data includes information added thereto for compensating the decreased number of high frequency components. Contrary to this, when the motion vector has no component of ½ pixel, the residual data includes no information added thereto for compensating the decreased number of high frequency components.
An objective of the present invention is to satisfactorily reduce an encoding noise (encoding distortion) of an image signal obtained as a result of decoding a motion-compensated predictive encoded-digital image signal.